The invention relates generally to apparatus and methods for igniting air/fuel mixtures in combustors. More particularly, the invention relates to the use of electrostatic atomization in such apparatus and methods.
A gas turbine engine is an example of an engine where ignition and engine restart can be a critical safety concern. For example, in aerospace applications, if a flame out occurs in an airborne jet engine, it may be necessary to restart the engine under extremely adverse conditions such as low ambient temperatures, thin atmospheric condition, and low fuel pressures as engine speed decelerates.
A combustor is a fundamental assembly used in turbine and other engines. The combustor typically includes a can or other annular casing that forms part or all of the combustion chamber. Within the combustor are one or more fuel nozzles which deliver fuel to the combustion chamber, along with air vents for delivering high pressure air to the combustion chamber. The fuel/air mixture is ignited near the region of the combustor closest to the fuel nozzles (i.e. the primary zone). The combustion process continues as the combusting fuel/air mixture moves down to the intermediate zone where additional air is supplied to cool the combustor wall and aid the combustion process. The process continues as the mixture of hot combustion gases enters the dilution zone where dilution air is supplied to cool the exhaust gases to protect the annulus casing from melting and downstream to protect the turbine blades. As is well known, homogeneity of the fuel burn within the combustion chamber is an important design criteria for a turbine engine.
Fuel delivery systems play an important part in the ability to initiate or restart a turbine engine. In known combustors, the fuel nozzles typically include a primary orifice and one or more secondary orifices. The purpose or the nozzle is to initially provide a fine fuel spray that can be ignited for engine start. After combustion starts and the engine speed increases, the secondary orifices are opened to increase fuel flow for engine idle and full throttle conditions.
The ease with which fuel can be ignited in the combustor depends on several key factors including fuel temperature, the type of igniter used, amount of ignition energy delivered, point of ignition energy delivery and the degree to which the fuel is atomized by the nozzle via the primary orifice. The atomization process is also important with respect to the overall efficiency of the fuel combustion.
Known aerospace gas turbine atomizing fuel nozzles include fuel pressure atomizers and air blast atomizers and combinations thereof. A fuel pressure atomizer uses a combination of high fuel pressure and an orifice to force atomization to occur. Fuel pressure at the orifice raises the energy of the fuel as it exits the nozzle, resulting in shearing of the liquid into small droplets. Droplet sizes are distributed in the form of a bell shaped curve. Thus, there will be large and small droplet size distributions around an average size droplet. The size distribution affects combustion because the larger the droplet size, more energy is needed and the more difficult it is to ignite and burn. Also, if the droplet sizes are too large, or if the air/fuel mixture is fuel rich, either condition will result in low burn efficiency and incomplete combustion. Incomplete combustion of the fuel produces black smoke (i.e. soot.) Increased levels of soot production cause a variety of operational problems for gas turbine engines (e.g. plug fouling, higher gas flow temperatures and increased infrared signatures). Fuel pressure atomizers must also have an operating pressure that can overcome the pressure build up that occurs in the combustion chamber. When flame out occurs, fuel pressure and air flow deteriorate rapidly, affording very little time to restart the engine. This is further exacerbated when the flame out occurs at thin atmospheric altitudes, creating a very lean operating environment.
Air blast atomizing nozzles use air pressure to atomize the fuel. Typically, such nozzles include an annulus for high speed air. The high air velocity provides the energy required to atomize the fuel stream into small particles. The air blast atomizer thus does not require high fuel pressures. However, the need for high speed air makes the air blast nozzle less than ideal for engine restart at high altitudes.
Low temperature ambient conditions present further difficulty for ignition and restart using conventional nozzles. This is because at low temperature the fuel viscosity can increase substantially, thus making atomization more difficult.
Combustors also require an igniter device to initiate the combustion process. Known igniters are plasma type spark plugs and glow plugs. Typically, the spark plug is mounted in the combustor wall near the fuel nozzle. In a conventional combustor, the primary zone or optimum region for ignition is the high turbulence region just forward of the nozzle outlet. However, the igniter cannot protrude down into this optimum region because it would be destroyed by the fuel combustion process. Retractable igniters are sometimes used with furnaces, but are not deemed reliable for aerospace applications. Thus, particularly in aircraft engine combustors, the igniter is mounted in a recess on the wall of the combustor near the primary zone. A high energy plasma, high temperature spark kernel is created at the periphery of the combustor wall and protrudes into the combustion chamber. However, there are numerous disadvantages including the fact that the combustor wall tends to act as a heat sink and quenches the intensity of the spark. The fuel/air mixture also is not optimum in this region. Obviously, the combustors are designed so that this type of ignition arrangement works, but it is less than ideal.
A known alternative to the spark kernel is the use of a torch burner which creates a flame that is used to ignite the main fuel supply in the primary zone of the combustion chamber. Known torch burners, however, still produce less than ideal results because of their reliance on conventional fuel supply nozzles and orifices. Under adverse conditions such as low temperature and high altitude they can experience relight difficulties.
Conventional plasma type spark plugs are commonly used for igniters. Unfortunately, by their very nature of using high voltage/current plasma discharge, they exhibit considerable electrode degradation and must be routinely replaced. Also, less than optimum combustion, particularly during engine start up and shut down, and/or fuel exposure, can produce plug fouling which degrades the spark discharge intensity or can prevent ignition. Varnish and other combustion by-products, particularly due to incomplete combustion and fuel evaporation, also can deteriorate plug performance. As a result, very high energy must be delivered to the spark plug to insure that carbon and fuel deposits are literally blown off the electrodes to produce an adequate spark. This excess energy, however, causes more rapid degradation of the electrodes, thereby shortening their useful life and increasing maintenance. Furthermore, the high energy required to produce the spark is typically supplied from an exciter circuit, such as a capacitive or inductive discharge exciter. The exciter circuit is located remote from the combustion chamber, however, due to the associated electronics. Consequently, the exciter must be connected to the plug by way of long coaxial cable leads or wires. This wiring causes many problems, not the least of which is simply energy loss. For example, to produce a two joule discharge at the plug, the exciter circuit may be required to produce ten joules of power, resulting in low ignition system efficiency, hence higher weight and cost.
The need exists, therefore, for better and more reliable and more efficient apparatus and methods for initiating combustion, particularly for engine restart under adverse conditions. The need also exists for an improved igniter that does not have the problems associated with conventional plasma type plugs.